Heat exchange conduit

ABSTRACT

A heat exchange conduit is disclosed by which there are achieved improved heat transmission conditions by the fact that a rotation of the flowing medium in the conduit is produced at the same time as said rotation is disturbed by means being in good heat conducting connection with the wall of the conduit. The heat exchange conduit comprises internal ribs extending parallel to the longitudinal axis of the conduit, and at least one additional element in the conduit for promoting the heat exchange, whereby at least one such additional element is constituted by a strip, each strip being twisted and its principal axis being parallel to the longitudinal axis of the conduit.

United States Patent [191 Kleppe et al.

1 1 Mar. 11,1975

I 1 HEAT EXCHANGE CONDUIT [73] Assignee: A/S RaufossAmmunisjonsfabrikker,

Raufoss, Norway [22] Filed: Feb. 9, 1973 [2]] Appl. No.: 331,248

[30] Foreign Application Priority Data 3,343,250 9/1967 Berto et a1.138/112 FOREIGN PATENTS OR APPLICATIONS 0,362,995 4/1906 France 165/179Primary Examiner-S Clement Swisher Assistant E.\'aminer-Daniel M. Yasich[57] ABSTRACT A heat exchange conduit is disclosed by which there areachieved improved heat transmission conditions by the fact that arotation of the flowing medium in the conduit is produced at the sametime as said rotation is disturbed by means being in good heatconducting connection with the wall of the conduit. The heat exchangeconduit comprises internal ribs extending parallel to the longitudinalaxis of the conduit, and at least one additional element in the conduitfor promoting the heat exchange, whereby at least one such additionalelement is constituted by a strip, each strip being twisted and itsprincipal axis being parallel to the longitudinal axis of the conduit.

3 Claims, 5 Drawing Figures 1 HEAT EXCHANGE CONDUIT The presentinvention relates to a heat exchange conduit comprising internal ribsextending parallel with the longitudinal axis of the conduit, and atleast one additional element disposed within the conduit in order topromote the heat exchange.

Such heat exchange conduits are utilized in heat exchangers wherein heatis to be transmitted from one medium to another through the wall of theconduit or tube. In order to increase the heat transmission inside suchtubes, the flowing medium within the tube has a turbulent flow. Thiscan-be achieved by the combination of sufficiently large diameter andflow velocity, and sufficiently low viscosity'for the flowing medium.

In many cases difficulties are encountered in achieving turbulence. Itmay be the case that a given viscosity may not be changed, or that thevelocity may not be above a given limit. In order to have transmitted acertain amount of heat per unit of time, and with given temperatures,the solution insuch cases generally is to increase the internal surface.Fora given tube dimension the surface may, for example, be increased bymeans of internal ribs in the tubes. Such ribs also are assumed to havea favorable influence on the heat exchange because the ribs projecttowards the interior of the tube and come into contact with thoseportions of the flowing medium which have temperature differential withthe outer layer. Measurements have shown that such ribs, in one case,increased the flow of heat through the tube wall approximately 14percent in relation to tubes without ribs, under otherwise equalconditions.

Another known device whose purpose is to give an improved heattransmission, is one or more twisted strips within the tube. This isshown in US. Pat. No. 1,343,352 and Swiss Pat. No. 353,387. Such stripshave the effect that the flowing medium circulates during the flowthrough the tubes, but the effect, with respect to the heattransmission, is not especially good. This is due to the fact that suchcirculation does not give any substantialoverturn of the flowing liquidlayers. The flow does not become turbulent for promoting heattransmission. Tests have shown that insertion of a twisted strip with awidth equal to the tube diameter, in a special case increased the flowof heat through the tube wall with less than 1 percent.

An object of the present invention is to provide a heat exchangecounduit in which there are achieved improved heattransmissionconditions by the fact that there is produced a rotation of the flowingmedium in the conduit, at the same time as this rotation is disturbed bymeans in good heat conducting connection with the wall of the conduit.

According to the invention, the above object is achieved by theprovision of a heat exchange conduit of the type set forth above, andwhich is caracterized in that at least one such additional element isconstituted by a strip, said strip being twisted and its principal axisbeing parallel with the longitudinal axis of the conduit.

The present invention constitutes a combination of the above mentionedconstituents, namely internal ribs and at least one twisted strip. Thiscombination has the surprising effect that the heat transmission throughthe wall of the conduit is increased substantially more than with thesum of the individual increases which are caused by the ribs and twistedstrip, respectively. Tests have shown that the disclosed combination isable to give more than a20 percent increase of the flow of heat throughthe wall of the conduit in relation to a conduit with smooth walls andwithout twisted strips, under otherwise equal conditions.

The invention departs substantially from the prior art with respect tothe use of ribs and twisted strips, respectively. The idea behind theinvention is to create a rotation of the flow by means of at least onetwisted strip, and to disturb this rotation by means of ribs which arein good heat conducting connection with the wall of the conduit, butwithout increasing the resistance to flow beyond a certain limit. Thedisturbance of the flow could be achieved by, for example, surfacesextending completely or partly transversely to the flow direction, butthis would firstly give strongly increased resistance to flow, andsecondly the production of such tubes would be complicated. The conduitsor tubes, exclusive of strips, which are utilized according to thepresent invention, may be extruded integrally with the internal andpossibly also external ribs. It is, however, no presupposition that thetubes are extruded. There is nothing to prevent the ribs from beingproduced separately and fixed to the tube, or the ribs being formed, forexample, depressed or folded parts of the tube.

Further details of the invention will become apparant from the followingdescription taken in connection with the accompanying drawings in which,

FIG. I shows a transverse section of a heat exchange tube according to aknown embodiment,

FIGS. 2 4 show embodiments of heat exchange tubes according to thepresent invention, and

FIG. 5 shows a heat exchange tube according to the invention which isalso provided with external ribs.

The tube 1 shown in FIG. 1 is of the previously known type discussedabove, in which the internal surface of the tube is increased by meansof internal ribs to which are formed integrally with or attached to thewall of the tube and project towards the center of the tube.

In FIGS. 2 4 are shown embodiments of tubes according to the invention.A tube 1 with ribs 2 has at least one internal twisted strip 3 whichpreferably extends along the whole length of the tube. The tube withribs is preferably made of a material with good thermal conductivity,whereas this is not essential in the case of the strip, as the stripbears against the ribs only at certain points and consequently haslittle effect with respect to heat conduction. Therefore it isconceivable to make the strip of a non-metallic material, such as aplastic, when this is advisable in view of temperature, chemicalinfluence from the flowing medium or in other regards.

The tube may have sections other than the circular form, and the ribs 2need not necessarily project radially into the tube as shown in FIG. 2.As shown in FIG. 3 the ribs may be parallel, and as shown the tube mayalso contain more than one strip. The essential requirement is that eachstrip causes a rotation of the flow, and that the ribs disturb thisrotation.

As previously mentioned, it is convenient but not necessary, that thetube with its internal ribs may be produced by extrusion. The materialmay be any metal or a metal alloy, preferably with good thermalconductivity. The internal and possibly the external ribs which areformed by the extrusion, will necessarily be parallel with thelongitudinal axis of the tube.

The number of ribs is not decisive. The essential requirement is thatthere be a suitable distance between the ribs, in order that the flowingmedium shall be able to move into and out of the spaces between theribs.

A simple and cheap embodiment of the invention comprises a tube with afew internal ribs and one twisted strip centrally disposed in the tube,so that the edges of the strip bear against the ribs at certain pointsas shown in FIG. 2. The ribs may project radially into the tube and beuniformly spaced around the wall of the tube. The strip may besufficiently fixed by means of friction against the ribs, or possibly byfastening in other ways. i

It is possible that the strips may have such a width that they do notbear against the ribs. This is shown in FIG. 5. With a certain distancebetween strips and ribs, the produced rotation of the flowing mediumwill also be effective at the ribs. In such a case it is possible thatthe strip will be situated somewhat irregularly within the tube if it isnot sufficiently fixed. It is convenient for the strip to be anchoredand maintained fairly coaxial with the tube by placing it under tensionor supporting it at certain points by means of auxiliary means (notshown) inside the tube.

A conduit or tube according to the invention may have external ribs ofany type, in cases where this is appropriate in order to improve theheat transmission conditions outside the tube. An example of externalribs 4 is shown in FIG. 5.

The invention may be utilized for all types of heat exchange whereby atleast one medium flows through tubes, either for increasing the heattransmission inside the tube, or for maintaining a given transfer ofheat in spite of reduced velocity of flow or increased viscosity. With agiven flowing medium and given tube dimensions, the invention enablesthe velocity of flow, and thereby the loss of pressure, to be loweredwith maintenance of a given heat transmission, as compared to known heatexchange tubes.

We claim:

1. A heat-exchange element comprising an elongated tube having inner andouter surfaces, internal, longitudinally extending ribs projectinginwardly from the inner surface of said tube and having inner edges, andat least one additional element disposed within said tube for promotingheat exchange, said additional element being constituted by a twistedstrip extending longitudinally through the tube and having a principalaxis parallel to the longitudinal axis of the tube, said additionalelement being so dimensioned and disposed in the tube to havelongitudinal edges bearing at least at certain points against the inneredges of at least two internal ribs whereby said ribs and stripscooperate to produce flow disturbance of a fluid flowing in the tube toincrease heat transmission between said fluid and the wall of the tube.

2. A heat-exchange element as claimed in claim 1 wherein said internalribs project towards the center of the tube and a single one of saidtwisted strips is disposed between said inner edgesof said ribs.

3. A heat-exchange element as claimed in claim 1, wherein at least twoof said twisted strips are disposed in said tube adjacent to one anotherand bear against said ribs at said certain points along their lengths.

1. A heat-exchange element comprising an elongated tube having inner andouter surfaces, internal, longitudinally extending ribs projectinginwardly from the inner surface of said tube and having inner edges, andat least one additional element disposed within said tube for promotingheat exchange, said additional element being constituted by a twistedstrip extending longitudinally through the tube and having a principalaxis parallel to the longitudinal axis of the tube, said additionalelement being so dimensioned and disposed in the tube to havelongitudinal edges bearing at least at certain points against the inneredges of at least two internal ribs whereby said ribs and stripscooperate to produce flow disturbance of a fluid flowing in the tube toincrease heat transmission between said fluid and the wall of thetube.
 1. A heat-exchange element comprising an elongated tube havinginner and outer surfaces, internal, longitudinally extending ribsprojecting inwardly from the inner surface of said tube and having inneredges, and at least one additional element disposed within said tube forpromoting heat exchange, said additional element being constituted by atwisted strip extending longitudinally through the tube and having aprincipal axis parallel to the longitudinal axis of the tube, saidadditional element being so dimensioned and disposed in the tube to havelongitudinal edges bearing at least at certain points against the inneredges of at least two internal ribs whereby said ribs and stripscooperate to produce flow disturbance of a fluid flowing in the tube toincrease heat transmission between said fluid and the wall of the tube.2. A heat-exchange element as claimed in claim 1 wherein said internalribs project towards the center of the tube and a single one of saidtwisted strips is disposed between the free edges of said ribs.